Planar antenna design

ABSTRACT

An antenna design includes a plurality of radiating elements which radiate electro-magnetic energy, and feeders which feed the electromagnetic energy to the radiating elements. The feeders have a supply network substantially at the same level in the antenna thickness direction. In order to achieve a small antenna with adequate properties for radio link usage, the radiating elements are arranged next to the supply network in the thickness direction and include box horn antennas which have a step, characteristic of a box horn, in the plane of the magnetic field.

This application is the national phase of international applicationPCT/FI96/00455 filed Aug. 23, 1996 which designated the U.S.

FIELD OF THE INVENTION

The present invention relates to an antenna design, particularly, forradio link applications.

BACKGROUND OF THE INVENTION

Currently, radio links employ several frequency bands on VHF (30 . . .300 MHz), UHF (300 MHz . . . 3 GHz), SHE (3 . . . 30 GHz), and EHF (30 .. . 300 GHz) bands. Ever higher frequencies have been used becausemobile services have almost entirely used the lower frequency bands(below 3 GHz). Presently, many radio link systems operate in the 38 GHzfrequency range, which, at least initially, is the range for the antennaaccording to the present invention. As the principle of the antenna isnot in any way tied to frequency, the antenna design of the invention isintended for use in the micro and millimeter ranges.

Radiation characteristics required of radio link antennas are specifiedin international standards. For example, the ETSI (EuropeanTelecommunications Standards Institute) standard prETS 300 197 specifiesthe highest levels permitted to side lobe levels in the radiationpattern of a 38 GHz radio link antenna. Thus, the starting point ofdesigning radio link antennas is typically such that the antenna gainmust be higher than a specific minimum level, but also such that theside lobe levels remain lower than specific limits. The gain cannot,therefore, be increased indefinitely because it would increase the sidelobe levels accordingly.

Requirements set for radio link antennas are strict, and, on frequenciespresently used, the radiation characteristics specified in the standardshave successfully been fulfilled only with different kinds of horn pluslens or reflector antennas (parabolic antennas).

Apart from adequate radiation characteristics, antenna manufacturers andespecially antenna users (customers) desire physically small antennas.Particularly when the terminal point of the radio link is at thecustomer's site, it is important for the antenna to blend into thebackground as well as possible (i.e., fit into a small space).

Laws of physics largely determine the antenna cross sectional area. Inother words, the antenna must have a specific capture area or itsaperture must have specific dimensions. Instead, through structuraldesign, dimensions of the antenna in the thickness direction can bemodified. For example, the drawback of the aforementioned horn plus lensor reflector antennas is that these antennas cannot be made compact dueto their operating principle. In the aforementioned 38 GHz range, forexample, such antennas are at least on the order of 20 cm thick.

Small dimensions in the thickness direction can be obtained by planarantennas (a planar antenna refers to a design in which the feeders andreflector elements of the antenna are very close to one another in thethickness direction). Planar antenna designs are often based onmicrostrip technique, which results in an insufficient gain due to thehigh loss of the microstrip structure. Many planar antenna designs alsoshare the drawback of being narrow-band (required characteristics areonly obtained on a narrow frequency band). Some planar antennas alsohave the disadvantage of being unsuitable for mass production due to thevery strict dimensioning requirements on the higher frequencies usedtoday. Antenna manufacturers desire an antenna design that can be massproduced.

SUMMARY OF THE INVENTION

It is an object of the present invention to avoid the above drawbacks byproviding a new type of an antenna structure which is suitable for radiolink use, has sufficient radiation characteristics, is compact, and issuitable for mass production. These objects are achieved by an antennadesign of the invention, which has a plurality of radiating elements andfeeders.

Such an antenna has specific properties (such as allowing a planarstructure, low losses, and wideband operation) through a planar supplynetwork, and incorporates known box horns by radiation characteristicsthat obviate the above drawbacks as radiating elements. Relating to thepresent invention, by optimal dimensioning of the box horn in a waysuitable even for mass production, it is possible to set the radiationpattern null of a single radiating element to the direction where thearray factor indicates a side lobe for the antenna array. In thismanner, the side lobe of the antenna array can easily be eliminated,whereby the desired radiation characteristics can be obtained withoutdifficulty.

The present invention provides a planar design with good (adequate forradio link use) radiation characteristics, a simple structure, lowmanufacturing costs, and insensitivity to manufacturing flaws. Forexample, in the aforementioned 38 GHZ range, the antenna according tothe present invention is only approximately 4 cm thick, i.e., inpractice about one fifth of the minimum thickness of current radio linkantennas.

Even though the whole antenna is constructed, according to a preferredembodiment of the invention, by waveguide techniques, a planar structureis still obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention and its preferred embodiments will bedescribed with reference to the examples in the attached drawings, inwhich

FIG. 1 shows a perspective view of the antenna according to the presentinvention, which has 2×2 radiating elements;

FIGS. 2a-2c illustrate a supply network used in the antenna design ofFIG. 1;

FIG. 3a illustrates a curved divider of the waveguide T-junction;

FIG. 3b illustrates a divider of the waveguide T-junction in which thedivider has been optimized structurally from the divider of FIG. 3a;

FIG. 3c illustrates a divider of the waveguide T-junction that providesan asymmetrical power distribution;

FIG. 4 illustrates the basic structure of a known box horn;

FIG. 5 shows how the ratio of the amplitudes of different wave modes inthe box horn is dependent on the ratio of the box horn apertures;

FIG. 6 shows the illumination of the box horn aperture;

FIG. 7a shows the basic structure of a radiating element used in theantenna of FIG. 1;

FIG. 7b illustrates a cross-section of the radiating element of FIG. 1in plane H;

FIG. 7c illustrates a cross-section of the radiating element of FIG. 1in plane E;

FIG. 8 shows a supply network intended for a 16×16 element array; and

FIG. 9 shows an array of radiating elements designed for the supplynetwork of FIG. 8.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows an antenna according to the present invention. The antennacomprises two parts, part A1 which contains the supply network, and partA2 which is attached on top of part A1 and contains the radiatingelement array 10 which (due to reasons of clarity), in this example, hasonly four radiating elements RE next to one another in a compact manner(two in both planes). Each radiating element RE is a box horn with astep S in the plane of the magnetic field. A feed aperture leading tothe supply network is marked with reference mark FA. Both the antennaparts (A, and A2) may be, e.g., closed metal parts that have beenproduced, e.g., by casting (the manufacturing technique of the antennawill be described in closer detail below).

FIG. 2a shows a top view of the lower part (A1) illustrated in FIG. 1,i.e., the face which is placed against part A2. FIG. 2b shows part A1viewed in 15 the direction of line 2B--2B of FIG. 2a, and FIG. 2c, inthe direction of line 2C--2C. This case uses a rectangular waveguide asa feeder. Using a rectangular waveguide as a feeder is a veryadvantageous choice due to its simple structure and low losses. The morecomplicated the structure, the more expensive it is to manufacture, andin most cases, the more prone to manufacturing flaws. The waveguideincludes a slot 20 provided on the surface of part A1, and part A2 formsthe ceiling of the waveguide. It is advantageous to have as narrow awaveguide as possible to obtain as narrow as possible spacing betweenthe radiating elements (element spacing), and consequently, few sidelobes for the antenna array. Thus, a narrow waveguide is advantageousfrom the standpoint of operating and cut-off frequencies.

In the aforementioned 38 GHz range, a waveguide width of approximately 5mm can be chosen, whereby, e.g., waveguide WR-28 having the width of7.11 mm and height of 3.56 mm may be chosen for a standard waveguide(not shown) feeding the antenna. It is thereby possible to choose thedepth D of slot 20 provided in part A1 to correspond to the height ofthe waveguide being used. For the feeding waveguide, an extension 25 isprovided at the feed aperture FA. The extension forms a transition fromthe wider waveguide to the narrower.

The waveguide operates solely on the lowest mode TE₁₀. For example, inthe waveguide WR-28, the cutoff frequency of TE₂₀ mode is 60 GHz, andthat of the TE₀₁ is 42.13 GHZ, which means that these wave modes cannotpropagate in the waveguide when the antenna is used on 38 GHz.)

In a planar supply network according to FIGS. 2a-2c, the power suppliedfrom a common supply source (not shown) is divided by successiveT-junctions to different radiating elements. In the example of FIG. 2a,e.g., there are three T-junctions. One of them is marked by referencemark T, and the borders of the junction are indicated by broken lines.As a conventional T-junction has a high reflection coefficient in awaveguide, it is advantageous to employ a rounded divider 22, based on atriangular model, in the T-junctions of the supply network. Such arounded divider is based on a known divider, illustrated in FIG. 3a, inwhich the tip 23a of the triangular divider 23 has been made extremelythin. Such a divider, with rounded sides and a thin tip, provides a lowreflection coefficient. However, the design is sensitive to the positionof the center point (tip 23a) of the divider. As a result, it isadvantageous to use the rounded divider 22 described above andillustrated in FIG. 3b. As far as tip 23a is concerned, the ideal shapeof the rounded divider has been altered by making the tip less sharp andsturdier, thereby making the divider less prone to manufacturing flaws.Good matching can nevertheless be maintained.

If it is necessary to deviate from evenly feeding the antenna array dueto requirements concerning the antenna radiation pattern, the requiredpower distribution ratios can be obtained in the T-junction by shiftingthe divider 22 in the middle of the junction off the center line. Ifsuch an asymmetrical power distribution between the elements is desired,it must be implemented without creating phase difference between theelements. In the T-junction, the phase difference between output gatesincreases in proportion to distance that the divider shifts further awayfrom the center line. This phase difference equals the phase differenceobtained if the position of the input gate is shifted sideways. Thus,phase is determined by distance to the divider, as measured from theoutput gates. This means that the phase difference can be compensated byshifting the position of the T-junction feeder guide an equidistancesideways to the same extent. This is illustrated in FIG. 3c, in whichreference mark X denotes the distance of the sideways shift. As aresult, the divider may be located in the center of the T-junction, butthe feeder guide may be to the side in relation to the divider.

The matching of the power divider can further be improved by generatinga second reflection which cancels the reflection from the divider. Ifthe amplitude of the reflection that is purposely caused equals thereflection from the divider, and they have opposite phases, the totalreflection summed will be zero. A reflection can be generated in thewaveguide by placing an obstruction in it. In the example according tothe figures, a cancelling reflection has been generated with acylindrical tap 24 (as shown in FIG. 2c and FIG. 3b). The amplitude ofthe reflection can be affected by adjusting the height h of the tap, andby shifting the location of the tap (its distance from the powerdivider), it is possible to obtain a desired phase.

In addition to power distribution in the supply network, the waveguidemust be curved. In FIGS. 2a-2c, the waveguide has a plane E curve in awaveguide branch leading to a single radiating element (below, the planeof the electric field will be referred to as plane E, and the plane ofthe magnetic field will be referred to as plane H). The curve has beenimplemented by providing the slots with sloping bevels of substantially45 degrees. The bevels are denoted by reference numbers 21 in FIGS. 2aand 2b. Because this results in polarization that would otherwise havean opposite phase between adjacent radiating elements in plane E, a halfwavelength prolongation Δ has been provided on one side. This reversesthe signal to be cophasal with the signal of the adjacent element in theplane E. At the bevels, each feeder branch is coupled to the radiatingelement, i.e., part A2 has a hole in a corresponding location, which isthe "feed aperture" of the radiating element.

In the plane E, the spacing between the radiating elements is largelydetermined by the phase correction required. At least the T-junction andphase correction (Δ) must fit between the elements. On both sides, therewill be the curve in the plane E, and on the side where there is nophase correction, the curve cannot be placed right next to theT-junction because it disturbs the fields present in the T-junction. Toassure reliable operation, the distance between the T-junction and thecurve must in practice be at least one eighth of the wavelength.

The elements can be placed closer to one another in the plane H than inthe plane E. If the walls between the waveguides in the supply networkwere extremely thin, the element spacing would be d_(H) =2× thewaveguide width. In determining the spacing, it must, however, be noted(a) that the directivity (and therefore, gain) of the antenna array isat its highest when the element spacing is a multiple of 0.9λ (λ iswavelength in free space), and (b) that the number of side lobes of theantenna array is proportional to how many wavelengths the elementspacing represents. Thus, it is possible to increase the elementspacing, for example, to 0.9×2×λ, without increasing the number of sidelobes. The directivity of the antenna array, thereby, increases to itsmaximum with element spacings wider than a wavelength.

By design solutions described above (T-junctions, power dividers, andtap matching, which are known solutions), a person skilled in the art isable to dimension the supply network according to the operatingfrequency arid other requirements set for the antenna at any one time.As far as the invention is concerned, the essential matter concerningthe supply network is mainly its planar design and the possibility for alow-loss waveguide implementation. An advantageous detail is alsorepresented by the possibility to taper (referring to decreasing thesupply amplitude at the elements located at the edges of the array) theillumination over the antenna surface by dividers. The final supplynetwork is formed by placing the power dividers to obtain a desiredamplitude distribution for the radiating elements. Relative amplitudesof the elements are defined by computing the radiation pattern of theantenna array with different taperings. Due to the fact that taperingdecreases the gain and widens the main beam, it is advantageous to aimat maintaining the illumination function as close as possible to anevenly illuminated aperture.

As set forth in the above, the antenna design in accordance with theinvention uses a box horn as a radiating element. A box horn is a knownhorn antenna design, which has a greater directivity in the plane of themagnetic field (plane H) than does a conventional horn with an apertureof the same dimensions. The horn is constructed to generate a higherorder (third) wave mode having a phase which deviates, e.g., 180degrees, from the phase of the dominant mode in the antenna aperture.This higher order mode changes the aperture illumination (in the planeH) from a cosine type of an illumination towards one that more resemblesan even illumination or two cosine illuminations.

FIG. 4 illustrates the basic design of a known box horn. The horntypically includes a rectangular waveguide element 41, having length L.This part, which measures A in the plane H is referred to as a box. Thevalue of A must be high to allow higher order wave modes Te_(n0) (n=0 .. . 3) to propagate The horn is open at one end, and is fed from arectangular waveguide 42 at the other end. The feed can also be carriedout by a horn in the plane H (a waveguide whose aperture at the end hasbeen extended in the plane H direction, while keeping the dimensions inthe plane E unchanged). The feeding waveguide or horn, with an apertureA', is placed on the center line of the box in order to generate onlywave modes with an amplitude deviating from zero at the center of theaperture, i.e., TE₁₀ and TE₃₀ modes. The ratio between the amplitudes ofthese wave modes is dependent on the apertures ratio A'/A. Assuming thata₁ is the amplitude of the TE₃₀ mode and a₃ is the amplitude of the TE₃₀mode, their ratio can be presented as: ##EQU1##

Based on this dependence, the ratio between the amplitudes a₃ and a₁ canbe illustrated as a function of step height A'/A. This is illustrated inFIG. 5.

The amplitude distribution of the box horn aperture (in plane H) alsodepends on the ratio a₃ /a₁. FIG. 6 illustrates the amplitudedistribution with values 0-0.7 for the ratio a₃ /a₁. The horizontal axisrepresents perceptual distance from the aperture center point, and thevertical axis represents proportional level. It is assumed in the figurethat the phase difference between two propagating modes at the aperturelevel is 180 degrees. As the figure shows, the amplitude ratio value of0.35 provides a relatively good approximation for an even illuminationfunction, and the value of 0.55 for two cosine distributions. In theplane E, the field is evenly distributed in the waveguide, and the areaof the antenna aperture is evenly illuminated.

The antenna according to the present invention uses a box horn of thetype described above, and particularly, one which has a stepcharacteristic in the plane of the magnetic field. The step provides asimple means for changing the relative amplitudes of wave modespropagating in the horn.

The box horn for an antenna array according to the present invention isdesigned as follows. At first, the array factor is utilized in computingthe direction where the array factor indicates a side lobe. The arrayfactor, as known, is of the form: ##EQU2## where N is the number ofelements, and γ depends on the wavelength λ, element spacing d, and theangle of view θ, as follows:

    γ=kd sin (θ)+δ,

where the wave number k=2π/λ and δ represents phase difference betweenthe elements.

In order to compute the direction of the side lobe, element spacing andfrequency must be known. Element spacing is known based on the supplynetwork dimensions.

By computing the radiation pattern of the box horn for differentamplitude ratios, the amplitude ratio which has a null in the directionin which the array factor indicates a side lobe will be determined. Theradiation pattern of an aperture antenna is determined by the fieldpresent at the aperture. A Fourier transformation can be used incomputing the antenna radiation pattern when the field present at theaperture is known. Particularly, the radiation pattern can be defined asa Fourier transformation of the aperture distribution. Thus, if thefunction representing amplitude distribution is F(y), the radiationpattern can be computed as a function of angle φ in plane xy by theformula: ##EQU3## where β represents a propagation coefficient and L isthe dimension of the aperture in the measuring level. Hence, E(φ)represents a Fourier transformation of the function F(y).

After establishing the amplitude ratio at which the null of a singleradiating element occurs in the same direction where the array factorindicates a side lobe, the amplitude ratio can be used to define theaperture ratio A'/A this amplitude ratio. Based on the aperture ratio,the radiating element can be given its final measures, because based onthe ratio, the dimension of the step in the plane of the magnetic fieldis known. Accordingly, by using the size of the step, a desiredradiation pattern has been obtained, after defining the step positionwhich also has an influence on the result, for a single radiatingelement with a null in the direction in which the array factor indicatesa side lobe.

FIGS. 7a-7c illustrate the basic structure of a horn antenna 70,disclosed in FIG. 1 and used as a radiating element in the antennaaccording to the present invention. "Feed-throughs" matching the hornantennas will be provided in part A2. FIG. 7a shows a perspective viewof the radiating element, FIG. 7b shows a cross-section of the elementin plane H, and FIG. 7c, a cross-section of the element in plane E. Inthis example, the horn opens linearly in both the plane H and E. In theplane H. this holds true both prior to the step S (cf., face 71) andafter the step S (cf., face 72). In such a design, with changingdimensions in the plane H, the propagation factor of the wave changeswhen travailing from the step to the aperture level. A design with anenlargement in the plane H after the step has the advantage that theaperture of the radiating element can be made as large as possible andyet the walls between the radiating elements can have a specificthickness for reasons of processibility.

In the above, those principles have been described according to whichthe antenna of the invention can be designed to match requirements setfor it at any one time. By following the corresponding principles, theradiating element, for example, may be realized in a completelydifferent shape. The radiating element may, e.g., open nonlinearlymanner, or the enlargement may not be realized at all (this holds truefor both the plane E and plane H). As far as manufacturing technique isconcerned, the nonlinear enlargement is clearly worse than the linearlyopening radiating element described above.

The number of radiating elements may also vary according to requirementsset for the antenna. FIG. 8 shows a top view of a supply network for 256elements, corresponding to the view of FIG. 2a. The feed aperture FA ofthe antenna in this case is in the middle of the supply network. Asshown by the figure, the supply network in this case comprises 64 basicmodules illustrated in FIG. 2a. Each module has four parallel feedingbranches for four different radiating elements. In a preferredembodiment, the number of radiating elements equals a power of two(e.g., 2⁸ =256), because this results in a symmetrical antenna design.The number of elements required depends on the gain, size, and radiationpattern requirements set for the antenna.

In general, it can be noted that, if there are n radiating elements,power is divided in the supply network in (n-1) T-junctions so that eachelement is fed by a line having an equal electrical length, if theaforementioned phase correction is not taken into account. FIG. 9 shows(from above) part A2, analogous with part A1 of FIG. 8, which contains atotal of 256 radiating elements as in FIG. 7a.

In practice, the antenna design according to the invention may bevaried, e.g., in the following ways.

In the supply network, it is possible to use different kinds ofgenerally known matching methods and divider structures. The same holdstrue for dimensioning the waveguide. Wave lines other than a waveguidecan also be used.

The coupling of the signal from the supply network to the element can beimplemented in various ways, for example, through a probe, if amicrostrip is used.

The antenna can be manufactured from various kinds of conductivematerials, or by coating a suitable material with a conductive layer.Since the antenna is comprised of two closed parts, casting is, inpractice, a noteworthy manufacturing technique. The surfaces of theparts must be conductive and even, to work well. In addition,manufacturing methods exist in which the parts can be casted fromplastic and provided with a thin metal coating. Such a method is wellsuitable for mass production.

By using power dividers described above or other conventional powerdividers, it is also possible to influence the relative amplitude of asingle radiating element, and accordingly, shape the apertureillumination function as desired.

Although the invention is described above with reference to the examplesillustrated in the accompanying drawings, it is obvious that theinvention is not restricted thereto, but it may be varied within theinventive idea of the attached claims.

We claim:
 1. An antenna, said antenna comprising:a plurality ofradiating elements which radiate electromagnetic energy; and feederswhich feed said electromagnetic energy to said radiating elements, saidfeeders comprise a supply network substantially at the same level in anantenna thickness direction, wherein said radiating elements arearranged next to said supply network in said antenna thickness directionand comprise box horn antennas, said box horn antennas having a step inthe plane of the magnetic field.
 2. The antenna as claimed in claim 1,said antenna further comprisinga first part and a second part, saidsecond part being disposed on said first part, said first partcomprising said supply network and said second part comprising said boxhorn antennas.
 3. The antenna as claimed in claim 1, wherein said supplynetwork comprises waveguides, said waveguides having a substantiallyrectangular cross-section and in which power is divided to saidradiating elements by T-junctions.
 4. The antenna as claimed in claim 3,wherein at least some of said T-junctions being provided with atriangular divider, said triangular divider having a rounded tip toimprove matching.
 5. An antenna, said antenna comprising:a plurality ofradiating elements which radiate electromagnetic energy; and feederswhich feed said electromagnetic energy to said radiating elements, saidfeeders comprise a supply network substantially at the same level in anantenna thickness direction, wherein said radiating elements arearranged next to said supply network in said antenna thickness directionand comprise box horn antennas, said box horn antennas having a step inthe plane of the magnetic fields, wherein said supply network compriseswaveguides, said waveguides having a substantially rectangularcross-section and in which power is divided to said radiating elementsby T-junctions, wherein at least some of said T-junctions being providedwith a triangular divider, said triangular divider having a rounded tipto improve matching, and wherein at least in some of said T-junctions,said triangular divider, and said feeder guide being shifted sideways inrelation to each other so as to alter power distribution from an evendistribution.
 6. An antenna, said antenna comprising:a plurality ofradiating elements which radiate electromagnetic energy; and feederswhich feed said electromagnetic energy to said radiating elements, saidfeeders comprise a supply network substantially at the same level in anantenna thickness direction, wherein said radiating elements arearranged next to said supply network in said antenna thickness directionand comprise box horn antennas, said box horn antennas having a step inthe plane of the magnetic field, and wherein said box horn antennas openlinearly in the plane of the magnetic field at least after said step. 7.An antenna, said antenna comprising:a first planar element; a secondplanar element, said second planar element being mounted on top of saidfirst planar element, wherein said second planar element comprising aplurality of horn antennas for radiating electromagnetic energy, each ofsaid box horn antennas having a waveguide with an output and a feedingopening, said output opens to a top surface of said second planarelement and said feeding opening opens to a bottom surface of saidsecond planar element, wherein said first planar element comprising asupply network of waveguides on a top surface thereof, said supplynetwork feeds said electromagnetic energy to said box horn antennasthrough said feeding openings, and wherein each of said box hornantennas comprising a step-like change, said step-like change having adiameter in a direction parallel to the magnetic field of saidelectromagnetic energy.
 8. The antenna as claimed in claim 7, whereinsaid diameter of said box horn antenna waveguide increases linearly fromsaid step-like change to said top surface of the second planar element.9. An antenna as claimed in claim 7, wherein said supply networkcomprises waveguides, said waveguides having a substantially rectangularcross-section and in which power is divided to said radiating elementsby T-junctions.
 10. An antenna as claimed in claim 8, wherein saidsupply network comprises waveguides, said waveguides having asubstantially rectangular cross-section and in which power is divided tosaid radiating elements by T-junctions.
 11. An antenna as claimed inclaim 9, wherein at least some of said T-junctions being provided with atriangular divider, said triangular divider having a rounded tip toimprove matching.
 12. An antenna as claimed in claim 10, wherein atleast some of said T-junctions being provided with a triangular divider,said triangular divider having a rounded tip to improve matching.
 13. Anantenna as claimed in claim 11, wherein at least some of saidT-junctions, said triangular divider, and said feeder guide beingshifted sideways in relation to each other so as to alter powerdistribution from an even distribution.
 14. An antenna as claimed inclaim 12, wherein at least some of said T-junctions, said triangulardivider, and said feeder guide being shifted sideways in relation toeach other so as to alter power distribution from an even distribution.